A wide variety of applications arise where an unknown voltage must be compared to a standard voltage within certain preset voltage limits, or two separate voltages must be compared to determine, for example, which is the greater of the two. Such comparisons are used in, for example, feedback circuits, digital voltmeters, counters, etc.
Differential comparator circuits are commonly used for comparing or sensing the difference in levels (e.g., voltages) of a pair of input electrical signals. The differential comparator provides an indication of which of the two electrical input signal is larger.
U.S. Pat. No. 4,374,334 (W. E. Engler) which issued on Feb. 15, 1983 discloses a comparator comprising a first surface charge transistor and a second surface charge transistor which are provided in a common substrate. During a first phase of a cycle of operation, a first voltage signal is applied to a transfer gate of the first transistor and a second voltage signal is applied to a transfer gate of the second transistor. Additionally, during the first phase, the source region and the transfer gate of each of the transistors are equilibrated. The receiver regions or nodes of each of the transistors are precharged and then floated or isolated. During a second phase of the cycle of operation, the first voltage signal is applied to the transfer gate of the second transistor and the second voltage signal is applied to the transfer gate of the first transistor. Additionally, during the second phase, the receiver nodes accumulate an amount of charge proportional to the change of voltage resulting from the switching of the input signals. Such charge is received, however, only if a transfer gate becomes more positive (for n-channel transistors). The receiver node associated with the more positive signal in the first phase of operation will, therefore, remain charged while the receiver node associated with the more negative signal will be discharged, by transfer of charge, to ground. The extent of the potential change is proportional to the difference between the two voltage signals and the gain of the surface charge transistors.
A differential voltage comparator is disclosed in the book entitled "Electronic Circuit Design Handbook" by the editors of EEE magazine, published by Tab Books, revised 4th Edition, 1971, at pages 135 to 136. The disclosed differential voltage comparator provides a "go-no-go" indication when two input signals are compared. If the two input signals are within a preset differential voltage, then an output relay is not actuated and a "go" indication is provided. However, when the two input signals differ by more than the preset differential voltage, then the output relay is actuated and a "no-go" indication is provided.
The prior art differential comparator described hereinabove includes many transistors, diodes, resistors, thermal resistors, and a relay and, therefore, is a complicated circuit. Accordingly, it is desirable to have a comparator which operates at relatively high speeds and has relatively few components.